1. Field of the Invention
The present invention relates to the art of treating heavy metal-bearing aqueous solutions used in plating and metal finishing processes, and more particularly relates to the reduction or displacement of poisonous heavy metals such as copper from waste waters for preparing the waste water prior to discharge in municipal sewers.
2. Description of the Prior Art
Heavy metal plating is accomplished frequently by an aqueous solution or bath process which can be electrolytic or electroless. In almost all cases, high concentrations of the heavy metal are maintained. If the heavy metal is poisonous, such as, for example, copper (Cu), or is otherwise objectionable, problems are presented when excess solutions must be discharged. For example, copper and copper-bearing compounds are usually not permitted above limited concentrations in municipal sewer systems, and therefore the copper must be reduced below the concentration limits. In the case of metal hydroxides, for example, a common method of reducing concentration is precipitation. Hereinafter in this specification and in the claims, the term "precipitate" will be used to mean the separating in solid form of a substance from a liquid as the result of a chemical or of a physical change, as from the addition or subtraction of electrons or electron energies. An attempt is made to collect the precipitate in a filter and to deposit the filtrate in municipal dumps. This method of elimination is unsatisfactory and only partially successful, because the metal hydroxide dissolves and percolates back into the underground water table. Another method of reducing concentration amounts is to add very substantial amounts of water.
Heavy metal plating techniques are used often in the manufacture of printed circuit boards. Printed circuit boards and printed wire boards are manufactured in the United States principally by photographic techniques at the present time. These techniques involve the plating of the substrate fiberglass or silicon with an electrically conductive metal, frequently copper (Cu), which covers the entire circuit board. Photographic etching techniques are then employed to remove the copper from the areas of the board which are intended to be non-conductive in accordance with a circuit pattern predetermined by the design engineer.
The copper is removed by etching techniques which frequently call for covering the copper-plated circuit board only over those particular areas which are to be conductive, thus exposing the copper in the plated area which is to be removed. Acid or alkaline etchants are used to etch the exposed copper. The etched copper, very often in the form of copper sulfate (CuSO.sub.4), is rinsed away in solution by a flush or rinse of water. Hereinafter in this specification, the term water will be used to mean any liquid or fluid effluent comprising heavy metal or other contaminants, unless it is indicated that this means otherwise.
Copper is poisonous, and the discharge of water containing any substantial amount of copper in the municipal sewer system frequently subjects the manufacturer to stiff penalities. In addition, the environment systems of the watersheds and water bodies receiving the effluent are quite detrimentally altered. The customary method in the industry of removing the copper is to introduce great amounts of alkaline material as may be necessary. Alkaline material reduces the CuSO.sub.4 to copper-hydroxide (Cu(OH).sub.2). The Cu(OH).sub.2 is fluffy in water, and forms a slime on the bath chamber. Some solutions of CuSO.sub.4 and alkaline sulfate double salts such as KCuSO.sub.4.6H.sub.2 O may also be poisonous. The fluffy slime mentioned hereinabove concerns conservationists and environmentalists when it appears in the municipal effluent.
In its pure, anhydrous form, CuSO.sub.4 is a colorless salt which readily absorbs water to form the blue pentahydrate [CuSO.sub.4.5H.sub.2 O] known as "blue vitriol." Copper is very poisonous in all of its forms to low organisms, especially algae, and is used in controlled amounts in swimming pools and water works to prevent the growth of such organisms.
It has long been desired to have a process for removing such objectionable heavy metal from the bath water in industrial systems, and which reduce water requirements. It has further been sought to remove such objectionable heavy metal while leaving a substantially clear liquid having essentially a neutral pH without fluff, slime and similar debris, so that the substantially clear neutral and non-poisonous liquid can be drained into municipal sewer systems without endangering the environment or subjecting the plant to sever sewer fines.